During the use of plating baths, the carbonate content of the bath increases due to the action of the carbon dioxide contained in the air onto the alkali cyanides and alkali hydroxides, and the oxidation of said alkali cyanides.
In cyanide containing baths, in general a carbonate content of 25 to 40 grams/liter is useful. On the other hand, a carbonate content of above 60 grams/liter is harmful since
the throwing power decreases; PA1 the current efficiency rapidly decreases; PA1 the bath composition is to be continuously completed; and PA1 the quality of the metallic coatings is definitely reduced, inasmuch that a lustrous deposit is no longer obtained and the metallic coatings are rough and become gradually stained. PA1 (a) cooled in the open at temperatures slightly above or below the freezing point; or PA1 (b) cooled to +4 to 0.degree. C. in a separate crystallization device by means of cooling elements, e.g. by dipping containers filled with carbon dioxide ice into the bath, or by passing a cooling medium through cooling coils, if necessary with addition if crushed ice; thereafter the hard carbonate layers deposited on the cooling elements were mechanically removed (cf. T. W. Jelinek, Galvanisches Verzinken, page 93 -Saulgau (Germany) 1982-ISBN 3-87480-010-5). PA1 taking from said cyanide containing bath at least part of PA1 its bath liquid; PA1 stirring and cooling in a cooling container the token liquid while in the metastabile state until crystallization of the carbonates occurs; PA1 separating the crystallized carbonates from the liquid; and PA1 leading the residual liquor back to the plating bath.
So far, for removing said carbonates from the plating baths, the said baths were either
After separation of the removed carbonate, the residual liquor was again used in the plating baths.
The method (a) could not be tolerated under the environment legislation, and moreover could be executed only in winter. Other disadvantages were that the cooling generally had to be carried out uncontrolledly, so that often to much or to less carbonate was separated, and that at temperatures below the freezing there was the danger that the tub be burst.
The method (b), and particularly the mechanical removal the hard carbonate layers from the cooling elements, was complicated and expensive. Moreover, the separated crystallizate had to be waste disposed as a whole, since the carbonate components could not be re-dissolved.